Preliminary results of field studies on economic treatment levels for control of grape leafhoppers indicate that many growers in the San Joaquin Valley use insecticides unnecessarily because they lack accurate knowledge of insect population levels. Thompson Seedless grapes also appear to be more tolerant of nymph populations than expected. Results also have shown that some insecticides result in a severe build-up of spider mite populations, and further screening is necessary to find materials with least harmful side effects on beneficial parasites and predators.